Text and input

Part of the free LogicWiz Python course, Module 1: Start coding. The examples and practice run in the app.

Meher typed 2 samosas and 3 chais, and the app said she ordered 23 items. Nothing crashed. So what went wrong?

You'll learn to

• Make strings, join them with +, repeat them with *, and measure them with len()
• Read what the user types with input()
• Turn text into numbers with int() and float(), and back with str()
• Expect the surprises: 0.1 + 0.2, and // and % with negative numbers

Learn

Text is a string

Anything inside quotes is a string (text). "chai" and 'chai' are the same.

Use double quotes when the text has an apostrophe: "Meher's order".

"15" is a string too. It looks like a number, but to Python it is two characters, 1 and 5.

Joining, repeating, measuring

first = "Meher"
greet = "Hi " + first
print(greet)
print("ha" * 3)
print(len(greet))
print("Line 1\nLine 2")
print("-" * 20)

+ joins strings, * repeats them, len() counts characters (spaces count too). \n inside a string is a new line character.

input() reads what the user types

a = input("Samosas? ")
b = input("Chais? ")
print(a + b)
print(int(a) + int(b))

Here the box types 2 and 3 for you. a + b joined the text "2" and "3" into "23". int() turned them into numbers first, so the second line is 5.

input() always gives you a string, even when the user types a number. Wrap it: int(input()) for whole numbers, float(input()) for decimals like 12.5. In the practice tasks, use input() without prompt text so your output matches exactly.

Converting between types

price = float("12.5")
qty = int("4")
print(price * qty)
print("Total: Rs " + str(50))
print(int(7.9))
print(int("  42  "))

str() turns a number into text so you can join it. int(7.9) cuts off the decimal (7, it does not round). int("12.5") is an error: convert with float() first.

Decimals are slightly off

print(0.1 + 0.2)
print(round(0.1 + 0.2, 2))
print(7 // 2, -7 // 2)
print(7 % 3, -7 % 3)

Computers store decimals in binary, so 0.1 + 0.2 is a hair off. Round money before printing. // rounds down (towards minus infinity), so -7 // 2 is -4, and -7 % 3 is 2 so that 3 * (-3) + 2 = -7 still holds.

CBSE asks for the output of -7 // 2 (-4), "5" + "3" ("53") and "ab" * 3. Learn this error too: TypeError: can only concatenate str (not "int") to str. It means you tried "text" + number; use str(number) or a comma in print.

In one line

input() returns text; wrap it in int() or float() before doing maths, and str() a number before joining it to text.